Permselective membranes for gas separation are known and used commercially in applications such as the production of oxygen-enriched air, production of nitrogen-enriched-air for inerting and blanketing, the upgrading of natural gas streams to pipeline quality specifications (e.g., removal of carbon dioxide or nitrogen from raw natural gas), and the recovery of hydrogen from various petrochemical and oil refining streams (e.g., separation of hydrogen from methane, ethane, ethylene, or carbon monoxide). Preferred membranes for industrial gas separations exhibit a combination of high flux and high permselectivity. The separation of gases by polymeric membranes is thought to depend on the size of the gas molecules in relation to the molecular free volume within the polymer structure and the physical or chemical interaction of the gas with the polymer of the membrane. Many polymeric membranes exhibit attractive properties when the permeation performance is measured under ideal conditions; that is, using pure gas samples at relatively low temperature and pressure. Some industrial gas streams to which the membrane may be exposed are at elevated temperatures and/or pressures, and may contain components or impurities that exhibit strong interaction and reaction with the material of the membrane, which may ultimately swell or plasticize the material. Under certain conditions, the membrane may exhibit a reversible or irreversible reduction in flux and/or permselectivity, and ultimately, loss of membrane performance. A membrane with high flux and permselectivity for the gases of interest, and sufficient durability to sustain this performance after long-term contact with aggressive components in these streams under high pressure and temperature is highly desired.
One aspect of the present invention relates to novel gas separation membranes prepared from certain polyimides that possess an excellent balance of gas permeation rates and permselectivity for one gas over other gases in multi-component gas mixtures. Another aspect of the present invention relates to the surprisingly good durability of these when exposed to feed gas containing high levels of higher hydrocarbons. The term “higher hydrocarbon”, in the context of this invention, means a hydrocarbon molecule having at least five carbon atoms, and is composed of hydrogen and carbon atoms in straight chain, branched, cyclic or polycyclic configuration, and can be aliphatic or aromatic in nature. For the purpose of nomenclature, the term C5+ means a hydrocarbon of at least five carbon atoms, the term C6+ means a hydrocarbon of at least six carbon atoms, and the like.
Examples of industrial gas mixtures and processes that are amenable to membrane separation are well known in the art. Membranes have been most successful in applications where the feed stream is primarily bi-component and easy to separate, is relatively free from impurities that could adversely affect the membrane, and is at relatively low temperature and pressure. The economics of the membrane-separation process could be improved substantially if operated at higher temperatures and pressures, and if pretreatment of the feed stream to remove impurities could be eliminated or significantly reduced.
One of the first successful applications for membranes has been the recovery of hydrogen from the vent gas stream in ammonia plants where the feed gas is relatively clean and free of hydrocarbon impurities. Another successful application, again where the feed gas is relatively clean, has been for the removal of hydrogen from synthesis gas streams for adjustment of the hydrogen/carbon monoxide ratio.
Large quantities of hydrogen are consumed in oil refineries; and, much is wasted as fuel gas because it is difficult to recover. Membranes are ideally suited for the recovery of the waste hydrogen; however, most refinery streams contain trace levels of higher hydrocarbons, some of which may be close to their dew point or saturation level at operating conditions.
Membranes are typically two-sided and have a feed side, or a first side, and a permeate side, or a second side. Depending upon a variety of factors, including membrane structure, feed flow rate, and membrane integrity, in some circumstances either side of the membrane can be used for the first side and the second side. When a feed stream containing such components is processed by a membrane system, the hydrogen permeates through the membrane, and the gas on the feed side becomes more concentrated in the higher hydrocarbon components, to a point that may exceed the dew point. These components could condense on the membrane surface, or absorb within the membrane material, and result in swelling or plasticization of the material causing irreversible damage and change in performance. Even at lower contaminant concentration, membrane performance has been reported to decline, presumably due to strong sorption or interaction of the contaminant with the membrane material. In a general sense, aromatic hydrocarbons swell or plasticize polymeric membranes to a greater extent than aliphatic hydrocarbons, and thus must be essentially completely removed from the gaseous feed stream prior to contact with the membrane. Or alternatively, the materials of the membrane must be more resistant to sorption or interaction with contaminants in the streams.
Another application where membranes have had only marginal success is for the removal of carbon dioxide and acid gases from raw natural gas to achieve pipeline quality natural gas (essentially less than about 2.5% carbon dioxide). The major component of raw natural gas is methane, with lesser amounts of carbon dioxide, hydrogen sulfide, other sulfur-containing species, higher hydrocarbons, water, and nitrogen. The nature and purity of the raw gas is dependent on geographic location, geological formation, production history of the well, and the like. The majority of substandard raw gas is purified using chemical sorption systems, but these are costly to build, operate, and maintain. Membrane systems have had limited success in natural gas processing because of the need for extensive pretreatment systems to remove higher aliphatic and aromatic hydrocarbons prior to contact with the membrane. A membrane with improved resistance to hydrocarbons would greatly reduce or eliminate the need for extensive pretreatment, and thus make membranes a viable treatment alternative.
Thus, there remains a need for membranes that will maintain their performance in industrial streams that contain higher hydrocarbons, particularly aromatic hydrocarbons, that are present in refineries and gas fields.
Polyimide membranes are well known in the art. For example, U.S. Pat. No. 4,705,540 discloses highly permeable polyimide gas separation membranes prepared from phenylene diamines having substituents on all positions ortho to the amine functions and a rigid dianhydride or mixtures thereof, specifically pyromellitic dianhydride (PMDA) and 4,4′-(hexafluoroisopropylidene)-bis(phthalic anhydride) (6FDA). These polyimides form membranes with high gas permeabilities but fairly low permselectivities. These polyimides are also sensitive to various organic solvents.
U.S. Pat. No. 4,717,393 shows that polyimides incorporating at least in part 3,3′,4,4′-benzophenone tetracarboxylic dianhydride and phenylene diamines having substituents on all positions ortho to the amine functions can be photo chemically crosslinked. Membranes formed from this class of crosslinked polyimides have improved environmental stability and superior gas selectivity than the corresponding crosslinked polyimide. However, photochemical crosslinking is not truly a practical method for fabricating cost-effective gas separation membranes.
U.S. Pat. No. 4,880,442 discloses highly permeable polyimide gas separation membranes prepared from phenylene diamines having substituents on all positions ortho to the amine functions and essentially nonrigid dianhydrides. These polyimides again exhibit high gas permeabilities, but once again low permselectivities.
Bos et. al. (AIChE Journal, 47,1088 (2001)) report that polymer blends of polyimide Matrimid 5218 (3,3′,4,4′-benzophenone tetracarboxylic dianhydride and diaminophenylindane) and copolyimide P84 (copolyimide of 3,3′,4,4′-benzophenone tetracarboxylic dianhydride and 80% toluenediisocyanate and 20% 4,4′-methylene-bis(phenylisocyanate) can increase the stability of the membrane against carbon dioxide plasticization when compared to the plain Matrimid 5218 membrane.
Barsema et. al. (Journal of Membrane Science, 216 (2003), p 195–205) report the permeation performance of dense film and asymmetric hollow fiber membranes made from the copolymer derived from reacting benzophenone 3,3′,4,4′-tetracarboxylic acid dianhydride (BTDA) with a mixture of toluenediisocyanate and/or 4,4′-methylene-bis(phenylisocyanate).
U.S. Pat. Nos. 4,532,041; 4,571,444; 4,606,903; 4,836,927; 5,133,867; 6,180,008; and 6,187,987 disclose membranes based on a polyimide copolymer derived from the co-condensation of benzophenone 3,3′,4,4′-tetracarboxylic acid dianhydride (BTDA) and a mixture of di(4-aminophenyl)methane and a mixture of toluene diamines useful for liquid separations.
U.S. Pat. Nos. 5,605,627; 5,683,584; and 5,762,798 disclose asymmetric, microporous membranes based on a polyimide copolymer derived from the co-condensation of benzophenone-3,3′,4,4′-tetracarboxylic acid dianhydride (BTDA) and a mixture of di(4-aminophenyl)methane and a mixture of toluene diamines useful for liquid filtration or dialysis membranes.
U.S. Pat. No. 5,635,067 discloses a fluid separation membrane based on a blend of two distinct polyimides, one being the copolymer derived from the co-condensation of benzophenone 3,3′,4,4′-tetracarboxylic acid dianhydride (BTDA) and optionally pyromellitic dianyhdride (PMDA) with a mixture of toluenediisocyanate and/or 4,4′-methylene-bis(phenylisocyanate), and the other being Matrimid 5218.